support multiple corpus accounts (#334)

Add support for sharding across multiple storage accounts for blob containers used for corpus management.

Things to note:

1. Additional storage accounts must be in the same resource group, support the "blob" endpoint, and have the tag `storage_type` with the value `corpus`.  A utility is provided (`src/utils/add-corpus-storage-accounts`), which adds storage accounts. 
2. If any secondary storage accounts exist, they are used by default for containers.
3. Storage account names are cached in memory the Azure Function instance forever.   Upon adding new storage accounts, the app needs to be restarted to pick up the new accounts.

src/api-service/__app__/onefuzzlib/tasks/main.py

@@ -18,9 +18,9 @@ from onefuzztypes.webhooks import (
     WebhookEventTaskStopped,
 )
 
-from ..azure.containers import StorageType
 from ..azure.image import get_os
 from ..azure.queue import create_queue, delete_queue
+from ..azure.storage import StorageType
 from ..orm import MappingIntStrAny, ORMMixin, QueryFilter
 from ..pools import Node, Pool, Scaleset
 from ..proxy_forward import ProxyForward